This study will assess the addition of topotecan to the standard, first- line regimen of carboplatin and etoposide for the treatment of extensive stage SCLC. Based on its activity in SCLC and its novel mechanism of action, the addition of topotecan to carboplatin and etoposide may improve the response rate and median survival in extensive stage SCLD patients. The study objectives are as follows: 1) To evaluate the feasibility of targeting systemic exposure of topotecan when used in combination with carboplatin and etoposide in patients with extensive stage SCLC. 2) To define the maximum tolerated systemic exposure (MTSE) of topotecan in combination with carboplatin and etopside in extensive stage SCLC patients. 3) To assess the response rate, the time to disease progression, and overall survival of patients receiving carboplatin, topotecan and etoposide in extensive stage SCLC patients. 4) To correlate lactone and total topotecan clearance with glomerular filtration rate in this patient population.